Heat Treatment Method of a Ni-Based Superalloy for Wave-Type Grain Boundary and a Ni-Based Superalloy Produced Accordingly

ABSTRACT

The present invention suggests a method of heat treatment of a Ni-based superalloy that improves resistance against creep, fatigue and stress corrosion cracking while being economical and easy, and a Ni-based superalloy produced by using the same. The method and the superalloy of the present invention include solution treatment at the high temperature region during a heat treatment process after manufacturing or final cold working fabrication. Immediately following the solution treatment, the material is slowly cooled at 1˜15° C./minute down to the intermediate temperature region for aging treatment. After the slow cooling stage, aging treatment is directly performed by holding it at the intermediate temperature region for the prescribed time. Lastly, the aging treatment is followed by air-cooling stage.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119 to Application No.KR 10-2008-0056386 filed on Jun. 16, 2008, entitled “A Heat TreatmentMethod of a Ni-Based Superalloy for Wave-Type Grain Boundary and aNi-Based Superalloy Produced Accordingly,” the entire contents of whichare hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a heat treatment method of a Ni-basedsuperalloy and a Ni-based superalloy produced by using the same, andmore particularly, to the heat treatment method of a Ni-based superalloyimproving resistance against intergranular fracture caused by creep,fatigue, stress corrosion cracking, etc., and a Ni-based superalloy withwave-type or serrated grain boundaries.

BACKGROUND

Ni-based superalloy is used as a material for high temperaturecomponents such as power assembly for aero and industrial gas turbinesbecause it is excellent in formability, weldability, corrosionresistance, high temperature mechanical properties, etc. The material isexposed to harsh environment like constant or complex strain cycle dueto high temperature exposures and mechanical loads during operation, andends in failure caused by damage from creep, fatigue, stress corrosioncracking, etc. Therefore, improving resistance of the material againstmain damage mechanisms such as creep, fatigue, stress corrosioncracking, etc. has been an important issue to manufacturers, componentfabricators, operating companies, etc.

As illustrated in FIG. 1, the existing heat treatment processes appliedto manufacturing and processing of a wrought nickel based superalloy,NIMONIC 263, which is widely used for combustion lines for industrialgas turbines, transition ducts, etc. will be examined. The methodgenerally contains water-cooling (over 50° C./second) after solutiontreatment (over 5 minutes at 1000˜1200° C.) at the high temperatureregion. Then, after the prescribed time, the 2nd heat treatment processis applied by air-cooling after aging treatment (over 5 hours at700˜900° C.) at the intermediate temperature region. The above-statedheat treatment simply dissolves coarse carbides and γ′ particles intothe γ matrix at the solution treatment process after manufacturing orcold working, precipitates the carbides at grain boundaries in advanceat the aging treatment process, and simultaneously distributes the γ′particles uniformly within the matrix. Accordingly, the purposes are toenhance thermal stability of the material, decrease grain boundarysensitization, and improve high temperature strength of the material.However, this kind of heat treatment method cannot improve resistanceagainst creep, fatigue and stress corrosion cracking satisfactorily atthe present time. Therefore, a heat treatment method that is moreeconomical and simple while improving the resistance remarkably isrequired.

Korean Patent Publication No. 1999-024668 discloses the heat treatmentmethod of a Ni-based superalloy for improving corrosion resistance. Theabove-referenced patent suggested a heat treatment method in whichresistance of grain boundary fracture improves by changing the shapes ofgrain boundaries within the material to wavy shapes through slowing downthe cooling speed to 0.1˜5° C./minute in all the temperature range toroom temperature or in a certain range after solution treatment at thehigh temperature, and again treating with an agent. However, this methodis not economically efficient because the heat treatment takes too longtime since it cools the material in a relatively slow speed, and thegrain size becomes larger since the material is exposed to a hightemperature for a long time. In addition, γ′ particles become coarsenedand various harmful phases can be precipitated, therefore, althoughresistance against stress corrosion cracking might be improved, it candeteriorate tensile properties and high temperature mechanicalproperties like creep, fatigue, etc. Accordingly, it is deemed that themethod can be hardly applied to actual industrial spots.

SUMMARY

The present invention suggests a method of heat treatment of a Ni-basedsuperalloy that improves resistance against creep, fatigue and stresscorrosion cracking while being economical and easy, and a Ni-basedsuperalloy produced by using the same. The heat treatment method of aNi-based superalloy of the present invention to accomplish theabove-stated technical concerns includes producing or processing aNi-based superalloy and then, performing solution treatment at the hightemperature region during a heat treatment process. Immediatelyfollowing the solution treatment, the material is slowly cooled at 1˜15°C./minute down to the intermediate temperature region for agingtreatment. After the slow cooling stage, aging treatment is immediatelyperformed by holding it at the intermediate temperature region for theprescribed time. Lastly, the aging treatment is followed by air-coolingstage.

In the heat treatment method of the present invention, the above-statedslow cooling stage consists of three processes; the first process inwhich wave-type grain boundaries begin to form at some of flat grainboundaries made during the solution treatment; the second process inwhich some of the wave-type grain boundaries formed grow with stableamplitude and frequency while more wave-type grain boundaries form atthe flat grain boundaries; and, the third process in which planarcarbides begin to precipitate at the said some wave-type grainboundaries. In addition, in the aging treatment process, most of thewave-type grain boundaries formed grow into wave-type grain boundarieswith stable amplitude and frequency, and the carbides precipitated canstably grow in planar shapes with low interfacial energy on thewave-type grain boundaries.

More preferably in the present invention, the solution treatment isprocessed for the prescribed time at 1000˜1200° C. and the agingtreatment can be processed for the prescribed time at 700˜900° C.

The superalloy of the present invention for accomplishing the differenttechnical tasks has wave-type grain boundaries in which planar carbidesare arrayed apart from each other at the grain boundaries. At this time,the planar carbide shares the coherent interface with one grain whilesharing the incoherent interfaces with the opposite grain. The array ofincoherent interfaces of planar carbide particles formed at wave-typegrain boundaries is zigzag pattern.

According to the heat treatment method of a Ni-based superalloy and aNi-based superalloy produced using the same by the present invention, itis possible to improve resistance against intergranular fracture causedby creep, fatigue, stress corrosion cracking, etc., and at the sametime, to conduct a time- and cost-efficient heat treatment whilemaintaining basic properties of a Ni-based superalloy by leading toprecipitation of low-density carbides with low interfacial energy andimproving cohesive strength between the grain boundaries and the matrixthrough changing the shapes of grain boundaries to wave-type shapes.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when considering the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing illustrating the existing heat treatment process.

FIG. 2A is a drawing illustrating the heat treatment process of thepresent invention, while FIG. 2B is a drawing illustrating toconceptually explain changes of microstructures according to the processof FIG. 2A.

FIG. 3 and FIG. 4 are photos showing microstructures of NIMONIC263alloys resulted from the existing heat treatment method and the heattreatment method in the present invention, respectively.

FIG. 5 and FIG. 6 are photos showing the fractured surfaces aftertensile test conducted at room temperature of NIMONIC263 alloys resultedfrom the existing heat treatment method and the heat treatment method inthe present invention, respectively.

FIG. 7A and FIG. 7B are graphs illustrating a creep test resultconducted under 760° C./295 MPa and 815° C./180 MPa, respectively, usingNIMONIC263 alloys resulted from the existing heat treatment method andthe heat treatment method in the present invention.

DETAILED DESCRIPTION

Therefore, technical concerns the present invention intends toaccomplish are to improve resistance against creep, fatigue and stresscorrosion cracking, and to provide an economical and easy heat treatmentmethod of a Ni-based superalloy. In addition, another technical concernof the invention is to provide a Ni-based superalloy produced by usingthe method.

Exemplary embodiments of the present invention will now be describedbased upon the accompanying drawings. The following embodiments may bemodified variably, and the scope of the present invention is not limitedto those embodiments. The embodiments of the present invention areprovided to more perfectly explain the present invention to the skilledperson in the art.

Firstly, embodiments of the present invention will suggest the maindamage mechanism of a Ni-based superalloy and how to overcome thedamage, and further explain a heat treatment method embodying themethod. Here, main damage mechanism of a Ni-based superalloy like creep,fatigue, stress corrosion cracking, etc. is defined as grain boundarydamage to meet the convenience of explanation.

In case of grain boundary damage, a main damage mechanism of a Ni-basedsuperalloy, cracks mainly initiate and propagate along brittle grainboundaries. Accordingly, resistance against grain boundary damage can beimproved by reducing energy of the grain boundaries themselves,increasing the crack propagation distance, and changing morphology andcharacteristics of the carbides, that is, particles precipitated at thegrain boundaries. The embodiments of the present invention suggestreducing energy of the grain boundaries as mentioned above, increasingthe crack propagation distance, and changing morphology andcharacteristics of the grain boundary carbides through the formation ofwave-type or serrated grain boundaries.

The wave-type grain boundaries improve resistance against grain boundarydamage for the following reasons. First of all, it improves cohesivenesswith the matrix by reducing misorientation degree between two adjacentgrains, and makes crack propagation distance longer by changing grainboundary configuration. In addition, the carbides precipitated at thegrain boundaries become planar while having low-density and stabilizedlow interfacial energy. Although the planar carbides form at the samewave-type grain boundary, the preference of each carbide particle to onegrain selection for sharing coherency is alternating so that the arrayof incoherent interfaces of carbide particles formed at the wave-typegrain boundary is zigzag pattern.

As stated above, characteristics of the carbides are modified so as tobe favorable for resistance against grain boundary damage by forming thewave-type grain boundaries. That is, the density of incoherent interfacebetween the carbides and the matrix providing a preferential site forcavitation or crack formation becomes lower and stabilized so that theresistance against cavity or crack formation could be improved.Moreover, the zigzag array of incoherent interface of carbide makes itmore difficult for cavities or cracks to interlink to form anintergranular path for crack propagation; therefore, a lower rate ofcrack propagation along the grain boundaries.

Hence, the embodiments of the present invention suggest how to leadplanar carbide particles by forming wave-type grain boundaries.

Although there are various models regarding the formation of serrated orwave-type grain boundaries, recently, the present inventors have foundthat grain boundary serration occurs spontaneously in the absence ofcarbides as a result of the total free energy minimization of amaterial. That is, at the high temperature region, straight-line flatgrain boundaries develop in order to reduce the surface area term assmall as possible because the influence of surface energy is bigger thanmisorientation between two adjacent grains. The grain boundaries tend toserrate to have several wavy segments in order to lower theirinterfacial free energy at the intermediate temperature region where themisorientation term becomes more important than surface area term.

Considering this occurrence model of wave-type grain boundaries, thefollowing prerequisites are essential in order to form wave-type grainboundaries in a Ni-based superalloy of the present invention.

Firstly, carbide precipitation at the grain boundaries should beretarded as much as possible because of the following reasons; thecarbide particles may inhibit the boundary movement as pinning points,and it might be difficult to modify the carbide characters like densityand shape if the carbides form prior to the grain boundary serration.Thus, the supersaturation of carbon atoms should be suppressed.Secondly, sufficient temperature and time should be provided for thegrain boundaries to move largely; A thermal equilibrium state should becontinuously maintained during cooling from a higher solution to a loweraging treatment temperature, since grain boundary serration is known tooccur spontaneously.

In order to satisfy the above-mentioned prerequisites, the embodimentsof the present invention suggest holding a Ni-based superalloy for theprescribed time at the high temperature region in which the carbides aredissolved; slowly cooling down to under the intermediate temperatureregion in which misorientation between two adjacent grains is important;and then, immediately conducting aging treatment at the sametemperature. In addition, the method maintained basic characteristicsrequired by Ni-based superalloy while creating wave-type grainboundaries. Accordingly, the present invention suggests a new heattreatment method that is simpler than the existing heat treatmentmethods and corresponds with the purpose of the present invention.

The present invention suggests the optimum heat treatment conditionsthat lead wave-type grain boundaries while maintaining the grain sizeand the volume fraction of γ′ particles through heat treatment testswith various conditions. Detailed conditions include holding at the hightemperature region for the prescribed time for solution treatment;slowly cooling down to the intermediate temperature region for agingtreatment; immediately conducting the aging treatment at theintermediate temperature region; and then successively air-cooling. Atthis time, the slow cooling down to the intermediate temperature regionis performed at 1˜15° C./minute.

The heat treatment process of the present invention can be compared withprior methods as follows. The prior inventions applied a two-step heattreatment method in which solution treatment is processed at the hightemperature region (1000˜1200° C.), water cooling is conducted (over 50°C./second), and aging treatment is again performed at the intermediatetemperature region (700˜900° C.). However, the current invention is aone-step heat treatment method in which slow cooling is conducted downto the intermediate temperature region immediately after solutiontreatment, and then heat treatment is completed after holding ituntouched at the intermediate temperature region.

FIG. 2A is a drawing illustrating the heat treatment process of thepresent invention, while FIG. 2B is a drawing illustrating toconceptually explain changes of microstructures according to the processof 2A. Here, the heat treatment temperature and the heat treatmentduration are examples of representative conditions for heat treatment,but don't limit the range of the present invention. For this, a Ni-basedsuperalloy hot-rolled NIMONIC 263 was used.

Referring to FIGS. 2A and 2B, the heat treatment method of the presentinvention is divided into a solution treatment process (Step a), a slowcooling process (Steps b˜c), an aging treatment process (Step d) and anair-cooling process. That is, first of all, for the solution treatment,solution treatment duration, e.g. for over five minutes, is maintainedat 1000˜1200° C. of high temperature region. Then, the material iscooled slowly with a speed of 1˜15° C./minute to the intermediatetemperature region or the aging treatment temperature of 700˜900° C.Later, the aging treatment temperature of 700˜900° C. is maintained forover five hours, then, the heat treatment is completed afterair-cooling.

The solution treatment process is processed during the solution durationthat dissolves coarse carbides and γ′ particles sufficiently resultingin enough solution treatment of the superalloy in the present invention,but does not cause grain growth. At this time, grain boundaries of thematerial after the solution treatment or Step a are flat (20).

Wave-type grain boundaries begin to form during the slow cooling processwith a speed of 1˜15° C./minute to the intermediate temperature region.The grain boundaries begin to have wave-type partially at Step b calledthe early stage of slow cooling process. At this time, the amplitude andfrequency of the wave-type grain boundaries (22) at the early stage of aslow cooling process have not developed completely (this will bereferred to as incomplete wave-type grain boundaries for convenience).

On the other hand, in Step c, the incomplete wave-type grain boundaries(22) are forming continuously and some of them grow into completewave-type grain boundaries (24) with stable wave-type, which is calledthe late stage of a slow cooling process. Namely, incomplete wave-typegrain boundaries (22), complete wave-type grain boundaries (24) and someflat grain boundaries (20) with no wave-type coexist at the late stageof slow cooling process. At this time, planar carbides (30) begin toprecipitate at the incomplete wave-type grain boundaries (22) and thecomplete wave-type grain boundaries (24) and precipitation hardenedphase γ′ begins to form at the matrix. The planar carbide (30) sharesthe coherent interface with one grain constituting the wave-type grainboundary (22 and 24) while sharing the incoherent interface withopposite grain.

The slow cooling process is immediately followed by the aging treatmentprocess, and after a prescribed time passes, most of wave-type grainboundaries grow to complete wave-type grain boundaries (24) as stated inStep d and the precipitated carbides (30) grow to form planar carbides(32). At this time, the carbides (32) grow creating an incoherentinterface to the direction of the opposite grain of the coherentinterface, while sharing the coherency with one grain. At this time, thearray of incoherent interfaces of the planar carbides is zigzag patternbecause of crystallographic variants of the wave-type grain boundaryitself.

The aging treatment process is processed during the aging treatmentduration in which sufficient aging treatment takes place securing nomicrostructural changes under exposure to the same aging treatmenttemperature region (700˜900° C.), by uniformly distributing γ′ particlesof the superalloy within the matrix and stabilizing the carbides at thegrain boundaries, coinciding with the purpose of the present invention.At this time, the planar carbides (32) grow stably at the completewave-type grain boundaries (24).

The planar carbides (32) with completed aging treatment process arearrayed away from each other by the wave-type boundaries (24). Althoughthe planar carbides (32) form at the same serrated grain boundary, thepreference of each carbide (32) to one grain selection for sharingcoherency is alternating so that the array of incoherent interfaces ofthe carbide particles (32) formed at wave-type grain boundaries iszigzag pattern.

In brief, the interfacial energy of the grain boundary itself can belowered significantly because of transformation from the flat grainboundaries (20) to the complete wave-type grain boundaries (24). Inaddition, the density of the carbides (32) precipitated on the wave-typegrain boundaries with low interfacial energy becomes lower whileincoherent interfacial energy of the carbides (32) significantly becomeslower because they grow to stable planar carbides. Further, the array ofincoherent interfaces of the planar carbides can be a zigzag patternbecause of crystallographic variants of the wave-type grain boundaryitself.

In the present invention, the reason why the slow cooling is limited to1˜15° C./minute at process to the aging treatment temperatureimmediately after the solution treatment is due to concern about thatbasic mechanical characteristics can be deteriorated since the grainsand precipitation hardened γ′ phase coarsen as the exposure time becomeslonger in case that the cooling speed is under 1° C./minute. Inaddition, if the cooling speed exceeds 15° C./minute, it is impossibleto obtain wave-type grain boundaries because carbides are precipitatedfirst since there is no enough time for the grain boundaries totransform into wave-type grain boundaries.

On the other hand, in case that the material is slowly cooled at 1˜15°C./minute in the entire temperature region from the solution treatmenttemperature to room temperature after the solution treatment, thematerial cannot be used as it is and requires separate aging treatmentcausing extra time and cost because γ′ precipitation and thermalstability are not sufficient. If the material is slowly cooled at 1˜15°C./minute of the different temperature region from the aging treatmenttemperature of the present invention after the solution treatment, notonly wave-type grain boundaries do not form, but also aging treatmentshould be conducted again.

In addition, microstructure would be shown as Step c if the material iswater-quenched quickly after the slow cooling process suggested in thepresent invention. That is, incomplete wave-type grain boundaries (22),complete wave-type grain boundaries (24) and flat grain boundaries (20)coexist at this stage. The carbon atoms are at the state ofsupersaturation due to the water-quenching, so if aging treatment isconducted, granular shapes of carbides with high density areprecipitated at the incomplete wave-type grain boundaries (22), thecomplete wave-type grain boundaries (24) and even the flat grainboundaries (20). These cases have higher interfacial energy than thepresent invention.

<Experiment Examples>

FIG. 3 is a photo showing microstructures of NIMONIC263 alloys resultedfrom the existing heat treatment method. The below one is an enlargedphoto of near a grain boundary. Solution treatment was performed forabout 30 minutes at the temperature of 1150° C., and the material waswater-quenched to room temperature (over 50° C./second), and then, agingtreatment was conducted again for about 8 hours at the temperature of800° C., and then the material is air-cooled. As illustrated in thephoto, with respect to the microstructures of the existing alloy, it wasfound that small granular carbides are precipitated with high density atstraight-line flat grain boundaries. It was identified that the size ofgrains is 60˜70 μm.

FIG. 4 is a photo showing microstructures of NIMONIC263 alloys resultedfrom the heat treatment method in the present invention. The below oneis an enlarged photo of near a grain boundary. Solution treatment wasperformed for about 30 minutes at the temperature of 1150° C., andimmediately the material was slowly cooled down to the aging treatmenttemperature of 800° C. at the speed of 10° C./minute, and then thematerial is air-cooled after holding for 8 hours at 800° C.

According to FIG. 4, it was found that, in the microstructures of theembodiments of the present invention, wave-type grain boundaries arewell developed and planar carbides with low interfacial energy areprecipitated at the grain boundaries with low density. At this time, thesize of grains is 70˜80 μm which is similar to microstructures obtainedfrom ordinary heat treatment.

Characteristics of alloys obtained from the existing heat treatmentmethod as illustrated FIG. 3 and alloys obtained from the heat treatmentmethod in the present invention as illustrated FIG. 4 are to be examinedin the following.

[Table 1] is results of tensile test of each alloy conducted at roomtemperature.

TABLE 1 Yield Tensile Grain Size Strength Strength Elongation Sample(μm) (MPa) (MPa) (%) Alloy from the existing 62 640 1083 23.3 heattreatment Alloy from the present 75 622 1079 38.1 invention

As we know from the above table, an alloy from the present inventionpresented similar yield strength and tensile strength to an alloy fromthe existing heat treatment. However, it was found that elongationindicating ductility significantly increased from 23.3% of the existingalloy to 38.1%.

FIG. 5 and FIG. 6 are photos showing the fractured surfaces aftertensile test conducted at room temperature of NIMONIC263 alloys obtainedfrom the existing heat treatment method and the heat treatment method inthe present invention, respectively. At this time, heat treatment is asexplained above. As illustrated, it was found that the grain boundaryfacets of the existing alloy were separated easily and fractured withoutparticular plastic deformation.

However, as illustrated in FIG. 6 of the present invention, considerabledeformation such as dimples and shearing on the wave-type grain boundaryfacets were found although the fracture mode remains essentiallyintergranular. Hence, it was found that the alloy of the presentinvention is fractured through sufficient plastic deformation up to justbefore fracture. In another words, the alloy of the present inventionhas relatively stronger cohesive strength between the grain boundariesand the matrix than the existing alloy. This result may be considered asone of elements increasing ductility as stated in [Table 1].

In the concrete, in the present invention, the carbides (32) withcompleted aging treatment are planar carbides that exist away from oneanother on the stable wave-type grain boundaries (24). The planarcarbides (32) form by turns in a zigzag pattern (FIG. 4 a and FIG. 4 b)towards two adjacent grains, not the array of incoherent interfacesplaced in one direction by growing toward the only grain out of the twograins constituting wave-type grain boundaries (24). Therefore,characteristics of the carbides (32) can be changed so as to befavorable to resistance against grain boundary damage because thewave-type grain boundaries (24) form as stated above. That is, thedensity of incoherent interfaces between the carbides (32) and thematrix providing preferential site for cavitation or crack formationbecomes lower and energy becomes more stable, causing the lower rate ofgrain boundary cracking. Even though grain boundary cracking isinitiated, sufficient plastic deformation is made up to just beforefracture because crack propagation along the grain boundary throughinterlinking is delayed due to the incoherent interfaces in a zigzagpattern.

FIG. 7A and FIG. 7B are graphs illustrating creep test results conductedunder 760° C./295 MPa and 815° C./180 MPa, respectively, usingNIMONIC263 alloys obtained from the existing heat treatment method andthe heat treatment method in the present invention.

From FIG. 7A and FIG. 7B, it was verified that the heat treatment of thepresent invention leads to excellent creep properties regardless of testconditions. More concretely, creep rupture life increased from about 129hours to about 178 hours, and creep strain also increased from about 6%to about 11% in the test under 760° C./295 MPa. In addition, creeprupture life increased from about 181 hours to about 252 hours, andcreep strain increased from about 17% to about 20% in the test under815° C./180 MPa.

As the present invention may be embodied in several forms withoutdeparting from the characteristics thereof, it should also be understoodthat the above-described embodiments are not limited by any of thedetails of the foregoing description, therefore, various variations arepossible by a person of ordinary skill in the pertinent art within therange of technical features of the present invention.

1. A heat treatment method of a Ni-based superalloy for wave-type grainboundary in heat treatment stage after producing or processing aNi-based superalloy, comprising: a solution treatment process at thehigh temperature region; a slow cooling process at 1˜15° C./minute downto the intermediate temperature region for direct aging treatment afterthe solution treatment; an aging treatment process by holding it for theprescribed time at the intermediate temperature region for the agingtreatment immediately after the slow cooling process; and an air-coolingprocess after the aging treatment.
 2. The heat treatment method of theNi-based superalloy according to claim 1, wherein the slow coolingprocess comprises, a stage in which incomplete wave-type grainboundaries form at some of flat grain boundaries formed during thesolution treatment process; and a stage in which the incompletewave-type grain boundaries grow into stable wave-type grain boundaries,incomplete wave-type grain boundaries form at the flat grain boundaries,and planar carbides begin to precipitate at the wave-type grainboundaries.
 3. The heat treatment method of the Ni-based superalloyaccording to claim 2, wherein aging treatment process is characterizedby that; most of the incomplete wave-type grain boundaries transforminto stable wave-type grain boundaries; and the precipitated carbidesform incoherent interfaces by growing into planar shapes towards theopposite grain while being coherent with one grain constituting thewave-type grain boundary.
 4. The heat treatment method of a Ni-basedsuperalloy for wave-type grain boundary according to claim 1, whereinthe solution treatment is processed at 1000˜1200° C. during the solutiontreatment time, and the aging treatment is processed at 700˜900° C.during the aging treatment time.
 5. A Ni-based superalloy for wave-typegrain boundary wherein wave-type grain boundaries are included and theplanar carbides are placed at the grain boundaries away from oneanother.
 6. The Ni-based superalloy according to claim 5, wherein thecarbides create a coherent interface with one grain with the above grainboundary, and make an incoherent interface by growing toward theopposite grain.
 7. The Ni-based superalloy for wave-type grain boundaryaccording to claim 5, wherein the array of incoherent interfaces of theplanar carbides formed at the wave-type grain boundaries is zigzagpattern.